"I'm past the age when I can claim the noun 'kid' no matter what adjective precedes it. But tonight we sure showed them what a comeback looks like," he said to his supporters who repeatedly chanted, "Mac is back!"

Clinton, 60, who finished third in Iowa, faced opinion polls before the New Hampshire primary showing her trailing Obama by double digits.

However, she won the primary by three percentage points, with 39 per cent to Obama's 36 per cent.

"Over the last week, I listened to you and in the process I found my own voice," she told her supporters.

"Now together, let's give America the kind of comeback that New Hampshire has just given me."

The 'tears effect'

Christopher Hitchens, author and columnist for Vanity Fair magazine, told Al Jazeera that Clinton's win in New Hampshire is considered a "comeback" of her own.

"We had a wonderful dilemma," said D Taylor, the union's secretary treasurer. "It's been a very difficult decision.

"We understand we are going against the Democratic power establishment ... we are used to being underdogs."

Unite Here, the group's parent organisation, with 460,000 food service, gaming and other workers nationwide, also expressed support for Obama,

Obama is expected to travel to Las Vegas to appear at a rally on Friday, his local office said.

Democrats vote next in Nevada on January 19 before their January 26 showdown in South Carolina.

'Super Tuesday'

New Hampshire's primary is the second high-profile battleground, following Iowa, in the state-by-state process of choosing Republican and Democratic candidates for November's election to succeed George Bush as president.

The election race now heads into an intense month of campaigning culminating in the Super Tuesday nominating contests on February 5, when some 22 states pick presidential candidates.

Republicans will contest their next primary in Michigan on Tuesday, where Romney faces a possible must-win and McCain and Huckabee will also contend.